Gelatino-silver halide emulsions having low setting temperatures and method of coating same



i atenteci Oct. 24, 1950 UNITED STATES PAT T OFFICE l GELATINO-SILVER HALIDE EMULSAIONS HAVING LOW SETTING TEMPERA- TUBES AND METHOD OF COATING SAME I John Alfred Henry Hart and-Edward William Lee, Ilford, ,England, assignors to Ilford Limited,

Ilford, England, a British company NoDravving. Application June 27, 1945, Serial a No. 601,926. In Great Britain July 19, 1944 This invention relates to -photographic ma- -terials and particularly to the coating of sheets and other articles with photographic emulsions. Although flat sheet "material such as glass plates, film and. paper can usually be coated with photographic emulsions without difliculty by well known coating techniques in which a layer of emulsionis'applied from a roll r or the like, to

the surface of the plate,'film or pa er; there are certain cases in which this technique is not very convenient and it is useless if the surface of the material to be coated is irregular.

An alternative method of coating is to spray :the photographicemulsion on to thearticle to be coated, withthe aidofa compressed-air-operated spray gun. This method is. particularly {suitable where the surface to be coated is irre ular, but may also be used to coat flat surfaces. However, the method is not without certain difliculties. V

Thus an ordinary gelatino silver halide emulsion tends to set in'the spray gun unless the gun I is specially heated. A diificulty which arises when freliefs or vertical surfaces are to be coated, is

that a normal gela'tinosilver halide emulsion does .not set quickly enough onthe sprayed surface so that the emulsion flows after coating and thus Y gives rise to an irregular coating thickness.

Another difficulty is that when the surface to be coated is hydrophobic the sprayed emulsion wets the surface only slowly, again resulting in an uneven coating. As the spraying proceeds the spray wets the already coated droplets more readily than it wets the hydrophobic surface, so

that there is a tendency for the spraying to result ;in larger and larger droplets-forming on the sprayed surface. These eventually fuse together, "but at this stage the coating is usually undesirably thick. If the emulsion sets beforethe fusion of the drops the non-uniformityof thecoating persists. g

, According to the present invention, the above difficulties are overcome by. employing as the .ernulsion tobe sprayed 'a gelatino silver halide emulsion which contains a substantial proportion of an anionic wetting agent and a substantial proportion of a water-miscible organic liquid.

Preferably the emulsion contains from 2 to 8 Claims.

thereby to increase the viscosity of the emulsion.

The further addition of a water-miscible organicliquid, however, causes a reduction in the viscosity of the emulsion and also a reduction in the temperature at which the emulsion sets.

In accordance with this invention the proportions of Wetting agent and organic liquid included in the gelatino silver halide emulsion are such "that the final emulsion has a setting temperature lower than that of a gelatino silver halide emul- 'sion. of similar character and gelatin content 7 which, does not contain the wetting. agent or organic liquid. The proportion of organic liquid which must be added is usually greater than that which would cause precipitation of the gelatin of the emulsion in the absence of an anionic wetting agent.

Thus emulsions which have been found very ferred) the emulsion sets very much more quickly than does an ordinary gelatinosilver halide emulsion. This is probably due to a combination of causes. Thus evaporation of the volatile solvent in the airblast from the spray gun causes rapid chilling of the emulsion, a rapid increase in the concentration ofthe gelatin and an increase in the relative proportion of water in the remaining 7 .-solvent mixture causing an increase; in the vis- 25% of an anionic wettingagent (based on the ;Weight of the gelatin); {The effectof adding an anionic wetting agent to-a gelatino,- silver halide emulsion is to cause the formation ofa gelatin/wetting agent complex (compare co-pending United States application Serial No. 600,844, filed June 21, 1945, nowrU, S. ratent 2, 3%) and cosity of the emulsion.

Ethyl alcohol, referred to above, is particularly suitable as the organic liquid but other watermiscible organic liquids may be employed, e. g.

methyl alcohol, isopropyl alcohol, methyl acetate,

acetone and dioxane. A mixture of organic liquids maybe employed. Although, in generaLit is desirable that the emulsion should set rapidly on the sprayed surface, where this is unnecessary or undesirable an organic liquid having a rate of evaporation similar to that of water may be used.

, ;Thus an emulsion containing dioxane sets less rapidly after spraying than does one which contains ethyl alcohol. 7

The quantity of organic liquid employed will I depend in each case on the characteristics of the emulsion used and on the nature and proportion of wetting agent present. Preferably it should be sufiicient to reduce the viscosity of the emulsion to a minimum without causing precipitation of the gelatin of the emulsion.

As examples of anionic wetting agents which may be employed there may be mentioned:

(a) Soluble salts of long-chain-alkyl carboxylic acids, e. g. soluble salts of fatty acids containing eight or more carbon atoms as, for example, oleic, ricinoleic, linoleic, stearic and palmitic acids.

(b) Soluble salts of long-.chain-alkyl sulphonic acids.

Soluble salts of sulphated higher fatty alcohols, e. g. soluble salts of fatty alcohol sulphates of which the alkyl group contains at least eight carbon atoms, as for example, the soluble salts of lauryl hydrogen sulphate and oleyl hydrogen sulphate.

:(d) Soluble salts of sulphated secondary alcohols containing at least eight carbon atoms in the chain.

(e) Soluble salts of alkylated aromatic sulphonic acids, e. .g. soluble salts of alkyl benzene sulphonic acids, of alkyl naphthalene sulphonic acids and of alkylated hydroxy diphenylsulphonic acids.

(7) isolubleisalts of lon'g-chain-alkyl esters of sulphated succinic acid.

The soluble salts may be formed from alkali metals, .e. g. sodium and potassium, from ammonia or from amines, e. g. triethanolamine and cyclohexylamine. The free acids may also be employed, where these are sufiiciently soluble.

' The compounds may contain amino residues in the anion of the soap :as, for example, in the sodium salt of oleyl amino ethane sulphonic acid which contains a carbonamide group =Very many anionic wetting agents are commercially marketed as detergents and these com- .mercial products may conveniently be employed in the process of this invention.

.If it is desired to ensure that the droplets of emulsion reaching the article to be sprayed should flow out to a uniform film immediately they arrive on the surface of the article, it is preferred to incorporate a wetting agent in the surface to be coated. The wetting agent may be incorporated in the substance of the article to be ';coated or applied to its surface in aqueous or non-aqueous solution by flowing, dipping, brush- Ling, beading or spraying. The wetting agent for this purpose maybe anionic, cationic or nonionic. :Sometimes it is desirabl 'to apply a preliminary coat (a' sub-coat) to the article to be sprayed in order to improve adhesion between-the article and the sprayed emulsion andthe wetting agent may be included in such sub-coat. A subcoat useful where the surface to be coated is hydrophobic, e. g. is a surface of cellulose nitrate or :methyl methacrylate polymer, may consist of .a hydrophobic dispersion of gelatin in an organic solvent medium. The choice of wetting agents suitable for addition "to such a sub-coat solution is limited to those free from basic radicles such as sodium, ammonium or cetyl trimetliylam- Y monium, which tend to fiocculate the gelatin.

Suitable wetting agents are, therefore, non-ionic agents such a; polyglycerol higher fatty acid esters or long-chain-alkyl sulphuric acids such :as l-methyl 'tridecyl sulphuric acid. When a .gelatino silver halide emulsion is sprayed :on to a white or light-coloured surface it is often difficult, using the usual photographic safe-lights, to see whether the emulsion is being sprayed uniformly, due to the low contrast between the sprayed emulsion and the surface which is being sprayed. This difliculty may be overcome by including in the emulsion to be sprayed a dy or pigment which absorbs strongly in the light which the safe-light filter transmits, so that the emulsion appears grey or black when viewed by the safe-light. It is of advantage to employ for this purpose a dye or pigment which is destroyed or dissolved away, or the colour of which is discharged during the usual hotographic processing operations.

The following examples of emulsions suitable for spraying serve to illustrate the invention:

ExampZeI Cc. Gelatino .silver halide emulsion containing gelatin 1,000 20% solution of sodium salt of l-methyl tridecyl. sulphuric acid 50 :Ethyl alcohol 1,000

7 The emulsion is heated to F. to melt it, the wetting agent 'is mixed in and the alcohol thenadded. The resulting emulsion is cooled to I "about 65 F. for use. Example 2 'Cc. Gelatino silver halide emulsion containing 8% gelatin 1,000 50% solution of 'triethanolamine dodecyl sulphate Acetone 500 Example 3 To 1000 cc. of an emulsion prepared as in EX- ample 1 there is added 20 cc. of .a 5%solution of Ink Blue, where the emulsion is to be sprayed in a yellow safelig'ht, or 20 cc. of a 5% solution of Acid Magenta, ,where the emulsion is to be sprayed in a green safelight. These dyes are readily removed during the normal development of the sprayed emulsions.

The following example illustrates a suitable sub-coat solution for use where the surface to be coated is hydrophobic.

Example 4 Gelatin -gr rm 10 .Salicyclic acid do 2.5 Rolyglycerol ester do 2.5 Water cubic centimeterm- 30 "Methyl alcohol do 1,000 Acetone do .l,000

25% :on the weight of the gelatin of an anion lsoa-p, 30 .70% :ofethe weight of the medium 'being water-miscible organic liquid.

2. A photographic emulsion comprising silver halide dispersed in a gelatin-anion soap complex made by admixing a gelatin silver halide emul-,

sion with an aqueous medium containing 2 to on the weight of the gelatin of an anion soap, -70% of the weight of the medium being ethyl alcohol.

3. A method of coating a, support with a photographic emulsion which comprises spraying the support at room temperature with a photographic emulsion comprising sliver halide dispersed in a gelatin-anion soap complex made by admixing a gelatin silver halide emulsion with an aqueous medium containing 2 to 10% on the weight of the gelatin of an anion soap, 30-70% of the weight 4. A method of coating a hydrophobic support with a photographic emulsion which comprises applying to the support a hydrophobic dispersion of gelatin in an organic solvent medium containing a non-ionic Wetting agent, allowing such coating to set and thereafter spraying at room temperature with a photographic emulsion comprising silver halide dispersed in gelatin in an aqueous medium containing a substantial proportion of an anionic wetting agent and a substantial proportion of a water-miscible organic liquid, the proportions being such that the emulsion has a setting temperature lower than that of an emulsion of similar strength in an aqueous medium containing no wetting agent or organic solvent.

5. A method of coating a hydrophobic support with a photographic emulsion which comprises applying to the support a hydrophobic dispersion of gelatin in an organic solvent medium containing a non-ionic wetting agent, allowing such coating to set and thereafter spraying at room temperature with a photographic emulsion comprising silver halide dispersed in gelatin in an aqueous medium containing a substantial proportion of an anionicwetting agent, 30-70% of the weight of the medium being water-miscible organic liquid the proportions being such that the emulsion has a setting temperature lower than coating to set and thereafter spraying at room temperature with a photographic emulsion comprising silver halide dispersed in gelatin in an aqueous medium containing 2 to 25% on the weight of the gelatin of an anionic wetting agent, 30-70% of the weight of the medium being watermiscibleorganic liquid.

7. A method of coating a hydrophobic support with a photographic emulsion which comprises applying to the support a hydrophobic dispersion of gelatin in an organic solvent medium containing a non-ionic wetting agent, allowing such coating to set and thereafter spraying at room temperature with a photographic emulsion comprising silver halide dispersed in gelatin in an aqueous medium containing 2 to 10% on the weight of the gelatin of an anionic wetting agent, 30-70% of the weight of the medium being ethyl alcohol.

8. A method of coating a support with a photographic emulsion whichcomprises spraying the support at room temperature with a photographic emulsion made by dispersing silver halides in a gelatin-anion soap complex obtained by adding to an aqueous gelatin silver halide emulsion 2 to 10% by weight, based on the gelatin, of an anion soap and adding to the emulsion 30 to by weight of such emulsion of ethyl alcohol.

JOHN ALFRED HENRY HART. EDWARD WILLIAM LEE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Wall, Photographic Emulsions, published 1929 by American Phot. Publ. Co., Boston, pp. -454. 

1. A PHOTOGRAPHIC EMULSION COMPRISING SILVER HALIDE DISPERSED IN A GELATIN-ANION SOAP COMPLEX MADE BY ADMIXING A GELATIN SILVER HALIDE EMULSION WITH AN AQUEOUS MEDIUM CONTAINING 2 TO 25% ON THE WEIGHT OF THE GELATIN OF AN ANION SOAP, 30-70% OF THE WEIGHT OF THE MEDIUM BEING WATER-MISCIBLE ORGANIC LIQUID. 